Radio receiving system



Sept. 6, 1938.

P. o FARNHAM RADIO RECEIVING SYSTEM 2 Sheets-Sheet l Filed 001;. l1. 1933 FIG. E.

gmantoz l Sept. 6, 1938. P, Qn FARNHAM 2,128,993

I RADIO RECEIVING' SYSTEM FIRE.

Patented Sept. 6, 1938 UNITED STATES PATENT OFFICE RADIO RECEIVING SYSTEM America, New York, N. Y., a corporation Delaware Application October 11, 1933, Serial No. 693,176

7 Claims.

This invention relates to radio receiving systems, .and particularly to receivers of the automatic control type.

The usual type of automatic control systems for radio frequency amplifiers includes a rectifier for developing a gain control voltage that varies with the amplifier input. Systems have been proposed in which the detector acts .as the source of rectied voltage for automatic gain control and, according to other proposals, the rectifier elements are included in the same tube with the elements of an amplifier or a detector. The present invention is well adapted for, and will be described as incorporated in, receivers employing a single tube housing the elements of the detector and rectifier, but certain of the novel circuit arrangements are equally applicable to receivers employing separate detector and rectifier tubes or equivalent elements.

To obtain a substantially constant output for a wide range of antenna input levels, it is necessary to delay the action of the automatic gain control system until the receiver input rises to that critical value which, for maximum amplifier gain, will produce a predetermined input level at the detector and, as the critical input value is exceeded, to rectify an .amplified radio output of the radio amplifier in a gain control circuit which gives a sharpincrease of direct current output for increasing values of the receiver input. The proposed arrangements for obtaining a rapid rise of the direct current gain control voltage with increasing receiver input voltage have usually necessitated the use of circuit elements and vacuum tubes additional to the gain control rectifier.

An object of the invention is to provide a simple and efficient gain control system in which a single tube and its associated network develop a direct current voltage for gain control purposes and an audio frequency voltage bearing a linear relation to the radio input to the tube, the audio voltage being of greater magnitude than could be developed by diode rectiiication of the radio input to the tube. An object is to provide a multiple function stage including a tube housing the elements of two diodes and an amplifier, radio input circuits to the diodes, a direct current output circuit for one diode, and an audio frequency output circuit for the other diode, the second diode working into the amplifier elements of the stage.

' Further objects are to provide methods of and circuit arrangements for improving the control characteristic of an automatic gain control receiver and, more particularly, to improve the characteristic by providing a substantially con (Cl. Z50-20) stant output over a wide range of input voltages with a sharp cut off of the gain control action when the input voltage falls below a predetermined critical level.

Another object of the invention is to provide an automatic gain control system including a diode rectiiier that is initially biased to prevent rectification for impressed radio voltages of less than .a predetermined value, and a second rectifier and direct current amplifier for automatically reducing the delay bias on the gain control rectier when the radio input rises above another and lesser predetermined value.

Other speciiic objects of the invention are to provide a combined detector-transmission control stage including a single tube which functions as a diode detector and an amplifier, the output circuit of the stage being adapted to transmit an audio voltage that is the sum of the output from the diode detector and from the audio amplifier. A further object is to provide a detector-audio amplifier circuit adapted to feed the two tubes of a succeeding push-pull stage through resistance couplings.

These and other objects and advantages of the invention will. be apparent from the following speciiication when taken with the accompanying drawings in which:

Fig. 1 is a circuit diagram of an embodiment of the invention in which the audio output of the detector-amplier tube may include either the sum of the diode detector output and the amplified detector output, or simply the ampliiied detector output;

Fig. 2 is a circuit diagram of another embodiment of the invention in which a resistance coupling is provided between the detector-amplifier stage and a succeeding push-pull stage;

Figs. 3, 4 and 5 are fragmentary circuit diagrams of other embodiments of the invention.

In the drawings, the reference numeral l identiiies a radio frequency amplifier of any convenient form and including one or more vacuum tubes for amplifying received signals at carrier and/or intermediate frequencies, the output circuit 2 of the ampliiier being coupled or connected to the plate circuit of the last amplifier tube. The tube 3 of the combined detector-control-amplifier stage includes .anodes A1, A2, a control grid G, plate P and a cathode K. The anodes A1, A2 cooperate with cathode K to form diode rectifiers and the coupling coils 4, 5 of the rectiiier circuits are coupled to the tuned output circuit 2 of the amplifier.

Considering iirst the detector diode Az-K, the

high potential terminal of coil 5 is connected to anode A2 and the low potential terminal is connected to the cathode through a resistance 6 which is by-passed for radio frequency currents by a condenser 'I. A direct current connection 8 extends from the anode terminal of resistance 6 to the control grid G and therefore the rectified voltage impressed' upon the input terminals of the amplifier includes both direct current and audio frequency components. The plate P is connected directly to a source of plate current supply, indicated by +B, and the audio frequency impedance in the plate circuit comprises the resistance 9 which is connected between the cathode K and the negative terminal -B of the plate current supply, which terminal is preferably a grounded point of zero radio and audio frequency potential.

The diode A1-K constitutes the rectifier element of the automatic gain control system and its coupling coil 4 is connected to the anode A1 and through a resistance Ill to ground. The direct current or output circuit of the diode rectifier therefore includes the resistance I and also the resistance 9 which is between the cathode and ground. This direct current circuit is by-passed for radio frequency voltages by the condenser II, and the lead I2 for transferring the automatic gain control voltage developed across the output resistance I8 to the amplifier I includes an alternating current filter formed by the resistance I3 and condenser I4.

The audio frequency voltage developed in the plate-cathode circuit of tube 3 across resistor 9, with or without the addition of the detector output audio voltage across resistor 6, is transferred through a blocking condenser I5 to a volume control voltage divider I6 by which it may be applied to the input of a succeeding audio stage. The direct current lead to the plate is by-passed for audio frequencies by condenser I'I and therefore the resistor 9 constitutes the entire plate circuit load. The audio voltage across the detector output resistor 6 is of the right phase to be added to the audio output developed across the resistor 9 and therefore the voltage divider I6 may be connected either to the grid or, if a lower audio output voltage is desired for the voltage divider I6, to the cathode terminal of the resistor 9. As illustrated in Fig. 1, a switch I8 may be provided for the alternative connection of the potentiometer I6 to a contact I9 which is connected to the grid or to a contact that is connected to the resistor 9.

The detector anode A2 is at the same direct current grid potential as the cathode K in the absence of incoming radio signals and therefore any radio voltage E impressed across the input coil 5 will be rectified by the detector to produce rectified direct and alternating current voltages across the resistance 6, these voltages being lmmediat-ely applied to the control grid G of the amplifier. The amplied audio frequency voltage developed across the resistor 9 is transferred to the potentiometer I6 which permits manual control of the audio output level of the receiver. The direct current voltage impressed upon the amplifier makes the grid G more negative and therefore decreases the voltage E1 which results from the flow of plate current through the resistance 9.

The radio input voltage Eo on the diode A1--K may be approximately equal to the input voltage E on the detector. In the absence of a radio input on the detector, the plate current flow in the amplifier is relatively high, and the voltage drop E1 across the plate circuit resistance 9 applies a high negative bias to the anode A1 of the gain control rectifier. It is apparent that this high negative bias Will prevent rectification until the peak radio input voltage En exceeds the bias voltage E1. As the radio input to the receiver increases, the rectification of the radio voltage E on the detector increases the control grid bias in a negative direction thereby decreasing the plate current flow and the delay bias voltage E1 on the gain control rectifier.

The gain control bias is developed across resistor I 0 only when the peak radio input En on the rectifier A1-K exceeds the delay bias voltage E1. Since the value of the delay bias E1 is automatically decreased with an increasing radio input to the receiver, there is a very rapid rate of increase in gain control bias for increasing values of receiver input voltage.

In the embodiment of the invention illustrated in Fig. 2, the same detector-control-amplfier action is effected but the output circuit of the stage is of different form to permit the feeding of Va succeeding push-pull amplifier stage through a resistive coupling. The circuit elements which are or may be identical with those of Fig. l have been identified by the same reference numerals and need not be described in detail.

To feed a push-pull amplifier through resistive coupling, the plate load resistance of the tube 3 is divided into two substantially equal sections 9'cl and 9b, the section 9a being located, as in the Fig, 1 circuit, between the cathode and ground, and the section 9b being connected between the plate P and the source +B of the plate current potential. The plate current source is by-passed by the audio frequency condenser II and therefore the entire audio frequency drop is developed across the serially connected resistances 9e, 9b. With reference to ground or zero potential, the audio frequency potential of the cathode terminal of the resistance 9et is equal to but of opposite phase to the potential of the plate terminal of resistance 9b. The plate and cathode terminals of these resistances may therefore be connected, through blocking c'ondensers 2| to the grids of a push-pull amplifier tube 22. A resistance 23 is connected between theV grids and center tapped for connection to the ground by a lead 24 to provide a direct current biasing path for the push-pull amplifier, the bias voltage being obtained, for example, by the potential dropacross a bias resistor 25 in the cathode circuits of the tubes. The eXtreme simplicity of embodiments of the invention will be evident from a comparison of the illustrated circuits with the usual circuit arrangements for an automatic volume control rectifier, a detector and an audio amplifier. While the use of a single vacuum tube contributes largely to the simplification of the circuits, it is to be noted that the desired cooperation which provides the sharp cutoff of the gain control at a critical radio input level is obtained Without the use of circuit elements additional to those essential for the normal and substantially independent action of the two diodes and the triode amplifier. Furthermore, the cathode bias resistor, 9 or 9e, performs additional and novel functions and eliminates the separate transformer or output resistance previously used in the amplifier plate circuit.

When, as is desirable, the audio output level is controlled between the detector and the first laudio amplifier, the circuit shown in Fig. 3 may be employed. As there shown, the anodes A1, A2 of the tube 33 are fed from the sections 34. 35,

respectively, of a split secondary coupled to the output circuit 2 of the amplifier I and this arrangement is to be understood as an alternative to the twin secondary windings shown in Figs. l and 2 in that it provides independent direct current connections for each diode and also permits application of different amplitudes of carrier voltage to the two diodes. The sections 34, 35 are separated by a blocking condenser 36 and are tuned by a condenser 31. So far as the circuit elernents are, or may be, the same as those previously described they are identified by corresponding reference numerals but will not be described in detail.

rl'he control grid is not permanently connected to the junction of the resistance 6 and winding 34 but an adjustable tap 38 is slidablez along resistance 5 to impress any desired portion of the available audio frequency potential upon the control grid G. The lead from tap 38 includ-es a filter resistance 39 shunted to the cathode by a condenser l to prevent the development of a radio potential upon the control grid and, if it is not desired to employ the described direct amplification of the gain control voltage, a blocking condenser 52 may be included in the lead to isolate the grid from the direct current voltage developed by the d-etector. The grid is connected to an intermediate point on the bias resistor 9 through a resistance 53, thereby placing an appropriate bias for amplification on the control grid. A condenser 44 is preferably connected across the upper section of resistance 9 to reduce d-egenerative effects in the audio amplifier and a condenser M is connected across resistor 9 to increase the audio component of the plate current output. The total direct current drop across resistor 9 serves as a delay bias on the diode rectifier of the gain control system.

Particular attention is directed to the fact that the tube 33 includes a static shield S between the diode elements and the amplifier elements of the tube, the shield being tied to the cathode K by a jumper s which is preferably located Within the tube itself. The static shield eliminates audio frequency couplings between the an.- odes A1, A2 and both the grid G and plate P, thus permitting a suppression of all audio output from tubey 33 when the tap 38 is adjusted to the cathode end of resistor 6. Except for this static shield, capacitive couplings within the tube would result in a substantial audio output even though the tap 38 is placed at the same audio potential as the cathode since the filter resistance 39 has a substantial impedance for audio frequencies. If the resistance 39 were replaced by a radio frequency choke, it would be possible to eliminate substantially all capacitive couplings to the grid, but there would then be a direct transfer of audio frequency voltage toplate P through the capacity between anode A2 and they plate. Successful operation of this type of output level control is therefore dependent upon an elimination or neutralization of capacitive couplings between the anodes and the amplifier elements of the multiple function tube 33.

As shown in Fig. 4, circuit arrangements similar to those shown in Figs. 1 and 2 may be used with a tube 45 to effect a grid circuit rectification and accompanying audio amplification of the signal, with a diode rectification in the gain control system. `The tube 45 is shown as a conventional screen grid tube but it is apparent that tubes such as shown in the preceding figures may be employed. The coupling to the circuit 2 of radio amplifier I includes a Winding 45 connected between cathode K and grid G1, and a winding 41 which is connected to the plate P and, through the output resistance l5 to ground or the negative terminal of the direct current source +B, which places a positive potential on the screen grid G2. The screen grid functions as the anode of the detector, and its output circuit includes the primary of an audio transformer 59 and, if desired, a resistance 50 which is shunted by an audio frequency by-pass condenser 5I. The purpose of these last elements is to eiect an automatic increase of the potential on G2 thus deferring over-V loading of the detector as the direct current voltage across the grid leak and condenser 52 increases with increasing radio input to the receiver. By appropriate choice of the value of the cathode bias resistor 53, which is shunted by the usual bypass condenser 54, it may be unnecessary to include the resistance 50 to obtain this result.

The automatic gain control bias for return to amplifier l is developed by diode rectification between the cathode K and plate P. The action of the gain control system is delayed by the negative bias established on plate P by the direct current potential developed across the bias resistor 53. With this type of rectifier, the direct current voltage developed across the grid leak and condenser 52 by rectification of the radio Voltage impressed on the detector controls the space current iiow, and in such sense that an increase in radio voltage reduces the space current flow, thus reducing the direct current drop across resistance 53. This reduction in space current reduces the delay bias impressed on plate P by the drop across the resistor 53, and therefore provides a more rapid ac, tion of the gain control system than would obtain if the delay bias on the rectifier were not reduced as the received carrier Wave increases in magnitude.

The circuit shown in Fig. 5 includes a multiple function tube 55 which has the same electrical characteristics as the tubes shown in Figs. l and 2, in that it includes a rectifier anode A1 and a detector anode A2 that are located outside the electron stream from cathode K to the` grid G2 which constitutes the anode or plate element of an audio amplifier. The amplified radio voltage at output circuit 2 is developed across a tuned circuit comprising inductance 55, tuning condenser 51 and blocking condenser 58, the radio voltage being directly applied to one anode, for example A1, and being applied to the other anode through a coupling condenser 59. By appropriate choice of the relative values of condensers 53 and la, it is possible. to place radio voltages of desired relative magnitudes upon the diode anodes Ai, A2. Any desired portion of the audio voltage developed across resistance 6 may be impressed on grid Gi through the adjustable tap 55, blocking condenser 6| an-d filter choke 62. The purpose of the choke 62 is to block radio voltages while passing to grid G1 the desired audio voltages selected by tap 60. The direct current flow through the cathode bias resistor 9 places a delay bias on the anode A1 which prevents the development of a gain control bias across resistance I0 until the receiver input exceeds a predetermined critical value. This delay bias is not removed, in the illustrated circuit, by the direct current amplifier action of the tube since the condenser 6I isolates the grid from the direct current voltage developed across resistance 6.

Each of the circuits herein illustrated and described ncludes a demodulator and a second rectifying device or element which yeffects an automatic control of the gain of the amplifier in accordance with variations in the strength of the signals to be received. It will be apparent, however, that the output of the second rectifying device or system may be employed to effect other types of automatic control, such as, for example, the actuation of a signal device or the control of the change-over relay when the receiver is part of a two-way transmission system.

It will be noted that there is considerable latitude in the design of the circuit arrangements and of the tube structures for carrying out the novel method of obtaining a relatively f'lat gain control characteristic witha sharp cut off at its lower end, and that various changes in these arrangements and structures fall within the spirit of my invention as set forth in the following claims.

This application is a continuation-in-part of and includes the subject matter originally described and claimed in my copending application Ser. No. 641,279, filed Nov. 4.-, 1932.

.I claim:

1. In a combined detector-amplifier stage, the combination with a vacuum tube having an anode, a control grid, a plate and a cathode, input and output circuits cooperating with said anode and cathode to form a diode detector, circuit elements for impressing between control grid and cathode the audio frequency voltage developed by said diode detector, a resistance between the cathode and the negative terminal of the source of plate current voltage, and a second resistance effectively connected between the plate and cathode in series with said first resistance, each of said resistances having a high impedance for voltage of audio frequency and of direct cur- Y rent, whereby the negative terminal of the plate current source and the non-adjacent terminals of said resistances constitute the three points across which a push-pull amplifier may be connected.

2. In a radio receiver, the combination with a radio amplifier, of a tube including the elements of two diode rectifiers and an amplifier, input circuit means coupling said tube to said radio amplifier to impress amplified radio voltages across each of said diodes, an output network for said diodes includingmeans across which audio and direct current voltages are developed, an audio output circuit for the amplier elements of said tube, circuit connections for impressing upon certain of the said amplifier elements the audio frequency voltage developed in said output network, and circuit connections for returning to said radio amplifier a gainv control bias derived from and varying with the direct current voltage developed in said output network.

3. The invention as claimed in claim 2, wherein said output network includes means across which one diode develops an audio Voltage and means across which the other diode develops a direct current voltage.

4. The invention as claimed in claim 2, wherein said input circuit means comprises a separate radio input circuit for each of said diodes.

' 5. In a radio receiver, a multiple function stage comprising a single tube having a cathode cooperating with two anodes to constitute diode rectifiers and with a grid and third anode to constitute an amplifier, Ya radio input circuit connected between said cathode and one of said diode anodes, a direct current output circuit between the said diode anode and said cathode, and a network of circuit elements connected between said cathode and the elements of said tube other than the said diode anode, said network including a radio input circuit and an audio output circuit in which said tube develops an audio voltage of greater magnitude than may be obtained by diode rectification of the radio voltage impressed across the'radio input circuit of said network.

6. The invention as set forth in claim 5, wherein the radio input circuit of said network is connected between said cathode and the second diode anode; said audio output circuit is connected between said cathode and the third anode; and said network includes a resistance across which an audio voltage is developed by the diode rectification action of said cathode and second anode, and circuit elements for impressing between the grid and cathode a desired portion of the audio voltage developed across said resistance.

7. In a radio receiver, the combination with a single tube having a cathode cooperating with a control grid and anode to function as an amplifier, and two additional anodes located outside of the space between said cathode and said amplifier anode, of input means for impressing radio voltages between each of said additional anodes and said cathode, an output network cooperating with saidY additional anodes and said cathode to form at least one diode rectifier circuit, said network including a resistance across which direct current and audio frequency voltages are developed, circuit elements for impressing between cathode and control grid a desired portion of the audio frequency voltage developed across said resistance, and an audio output circuit between said cathode and said amplifier anode.

PAUL O. FARNHAM. 

